1) Field of the Invention
This invention relates to a dryer vacuum box incorporated in a dryer part of a paper machine.
2) Description of the Related Art
A conventional paper machine employs a dryer part of, for example, (1) the single deck type wherein dryer rolls are juxtaposed in one stage or (2) the double deck type wherein dryer rolls are juxtaposed in two stages. Dryer parts of the types mentioned are described below.
(1) Single Deck Type
In a dryer part of the single deck type, a plurality of dryer rolls are disposed in an upper stage while a plurality of vacuum rolls are disposed in a lower stage, and a belt-like canvas is wrapped alternately around the dryer rolls and the vacuum rolls. A paper web (wet-web) dehydrated by an upstream press part runs alternately between and around the dryer rolls and the vacuum rolls while it is supported at one face thereof by the canvas. The paper web is supported on the lower face side of the canvas, and on each of the dryer rolls, the paper web is pressed to the surface of the dryer roll by the canvas and dried by heat of the dryer roll.
The paper web heated and dried by each of the dryer rolls advances to one of the vacuum rolls together with the canvas. When the paper web is spaced away from the dryer roll, sticking, such that the paper web sticks to the surface of the dryer roll sometimes occurs. The sticking is considered to be a phenomenon which occurs because paper powder sticking to the surface of the dryer roll acts as if it were paste, pasting the paper web to the surface of the dryer roll. If such sticking occurs, then the paper web is taken by the dryer roll side and spaced away from the canvas. Consequently, running of the paper web is disordered, and in the worst case, a break of the paper web occurs.
Therefore, the dryer part of a conventional paper machine includes a vacuum box such as shown in FIGS. 9 and 10 as a running stabilization apparatus for a paper web in order to suppress disorderly running of a paper web in a draw section from a dryer roll to a vacuum roll. FIG. 9 is a side elevational view partly in section showing a configuration of a conventional vacuum box, and FIG. 10 is a front elevational view of the vacuum box. Referring to FIGS. 9 and 10, two side seals 22 and 24 and two cross seals 26 and 28 are disposed among two adjacent dryer rolls 2 and 4 and a vacuum roll 10. The side seals 22 and 24 and the cross seals 26 and 28 are secured to and supported on a support pipe 40 disposed between the dryer rolls 2 and 4 and extending in a widthwise direction of the apparatus. The side seals 22 and 24 are shaped in conformity with the shape of a side section of a space defined by a portion of a canvas 6 from the dryer roll 2 to the vacuum roll 10, the vacuum roll 10, and another portion of the canvas 6 from the vacuum roll 10 to the dryer roll 4, and are disposed in a spaced relationship from each other by a distance substantially equal to the width W of a paper web 8 in the widthwise direction of the apparatus. The cross seals 26 and 28 are disposed in the widthwise direction of the apparatus between the side seals 22 and 24 and secured to a bracket 42 securely mounted at an upper end portion of the support pipe 40. One of the cross seals 26 and 28, that is, the cross seal 26, is secured to an end portion of the bracket 42 adjacent the dryer roll 2 and serves as a lid for a gap between the bracket 42 and the canvas 6 wrapped around the dryer roll 2. The other cross seal 28 is secured to an end portion of the bracket 42 adjacent the dryer roll 4 and serves as a lid for a gap between the bracket 42 and the canvas 6 wrapped around the dryer roll 4. An enclosed space 20 is defined by the side seals 22 and 24, cross seals 26 and 28, vacuum roll 10 and portions of the canvas 6, and the enclosed space 20 functions as a vacuum box 20 as a running stabilization apparatus for the paper web 8.
The inside of the vacuum roll 10 is partitioned into a plurality of chambers 10a, 10b, 10c and 10d in a circumferential direction by partition plates 14 as shown in FIG. 9, and the chambers 10a, 10b and 10c, along which the canvas 6 is wrapped, serve as vacuum chambers which are acted upon by vacuum suction force from a vacuum introduction path 12 disposed at the center of the vacuum roll 10. A plurality of holes are perforated in a surface cell of the vacuum roll 10 in such a manner as to establish a communication state between the entrance and exit side chambers 10a and 10c and the vacuum box 20 so that vacuum suction force acts in the vacuum box 20 from the chambers 10a and 10c. The inside of the vacuum box 20 is in a lower pressure state than the outside of the vacuum box 20 due to the vacuum suction force from the vacuum roll 10 so that, through the canvas 6 having air-permeability, suction force acts upon the paper web 8 accompanied by the canvas 6. The paper web 8 is constrained compulsorily to the canvas 6 by the suction force, and disordering of the running of the paper web 8 in the draw section from the dryer roll 2 to the vacuum roll 10 is suppressed.
It is to be noted that one apparatus which includes such a vacuum box as described above is disclosed particularly in Japanese Patent Laid-Open (Kokai) No. HEI 3-137288.
Meanwhile, in an apparatus disclosed in U.S. Pat. No. 4,905,380, a nozzle is provided in an opposing relationship to a draw section of canvas from a dryer roll (drying cylinder) to a suction roll in a dryer part of the single deck type, and air is jetted from the nozzle along the draw section so that a paper web may be sucked to the canvas in the draw section by an ejector effect of the air.
(2) Double Deck Type
In a dryer part of the double deck type, a plurality of dryer rolls are disposed at each of two stages of an upper stage and a lower stage, and canvas is wrapped around the dryer rolls. A paper web (wet-web) from a press part runs alternately between and around the dryer rolls in the upper stage and the dryer rolls in the lower stage while it is supported by the canvas. The paper web is heated and dried directly by the dryer rolls or indirectly through the canvas in the process of running around the circumferential faces of the dryer rolls.
In such a dryer part of the double deck type as just described, there is the possibility that sticking may occur when the paper web is to be spaced away from a dryer roll, and an apparatus disclosed as prior art, for example, in U.S. Pat. No. 4,876,803 is available as an apparatus which can prevent such sticking. The apparatus has a configuration such as shown in FIG. 11. Referring to FIG. 11, dryer rolls 160, 162 and 164 are disposed in an upper stage while dryer rolls 161, 163 and 165 are disposed in a lower stage. A paper web 8 is heated and dried by the dryer rolls 160 to 165 in a process of passage through while it is supported by canvas 170 or another canvas 171. In a loop of the canvas 170, blowers 190 and 191 are disposed between the dryer rolls 160 and 161 and between the dryer rolls 162 and 163, respectively.
Each of the blowers 190 and 191 jets air as indicated by an arrow mark A. Consequently, a negative pressure is generated in the loop of the canvas 170 by an ejector effect of the jetted air. As a result, the paper web 8 is sucked to the canvas 170 so that disorderly running of the paper web 8 can be suppressed to prevent otherwise possible sticking.
If sticking occurs, then the paper web 8 is taken by a greater amount by a dryer roll as the running speed of the paper web 8, that is, the paper speed, increases. Although development of a high speed paper machine which makes paper at a paper speed higher than 2,000 m/minute is proceeding these days, if sticking occurs with such a high speed paper machine as just described, then paper break occurs with a high probability, which causes deterioration of the availability of the paper machine and production of a large amount of paper loss. Particularly at such a very high paper making speed, which exceeds 2,000 m/minute, where such a vacuum box 20 as shown in FIGS. 9 and 10 is provided, since a negative pressure acts upon the vacuum box 20 through the vacuum roll 10, a sufficiently high degree of vacuum cannot be obtained. On the other hand, where such an apparatus as disclosed in U.S. Pat. No. 4,905,380 mentioned hereinabove or such blowers 190 and 191 as shown in FIG. 11 are provided, since a negative pressure acts due to an ejector effect, a sufficiently high degree of vacuum cannot be obtained. Consequently, in both cases, it is difficult to sufficiently suppress disorderly running of the paper web 8 caused by sticking.
Meanwhile, when a paper web is to be threaded through the dryer part of a paper machine, where the paper machine is o f the single deck type, referring particularly to FIG. 10, the paper web 8 is cut to form a tail 8a of a reduced width T and the tail 8a is threaded to a reel, whereafter the width of the paper web is increased to the full width W thereof. However, if sticking occurs and an end of the tail is taken by the dryer roll 2, then the paper web cannot be threaded any more, and consequently, it becomes required to perform a paper threading operation once again from the beginning.